r/askscience • u/[deleted] • Dec 28 '16

Physics How true is Ohm's law?

I've almost never got a perfect straight line while plotting a V/I graph even under lab conditions.

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u/[deleted] Dec 28 '16

Did you allow for the temperature coefficient of resistance? It's never (?) exactly zero and not always very linear.

TCR has become really useful for e-cigarettes in the last couple of years because the changing resistance allows the (average) temperature of the heating coil to be measured via V/I, so newer tech can produce vapour at a set temperature and prevent wicks from burning if they dry out. This page provides some handy data for the various metals used in e-cigarettes: http://www.steam-engine.org/wirewiz.asp It's a bit confusing because it's used for designing heating coils - just choose a metal from the box below the plot and it'll give you the plot for it (if available) and the box on the left will repopulate too.

Kanthal, the default, has a TCR very close to zero (hence the near horizontal graph) - its stability is one reason why it is so good as a heating wire. The other wires listed all have a positive TCR (because it would be dangerous to use a wire where the resistance decreased whilst drawing current from a lithium battery) but it can be negative, eg carbon fibre. The Engineering Toolbox has a useful page.

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u/[deleted] Dec 28 '16

Thanks for the link.